It is a state of the art solution to store fluctuating electrical energy from renewable energies as heat inside heat storages in order to reconvert it back to electrical energy in times when the demand is higher than the production. These heat storages are usually part of thermal energy storage plants which additionally can comprise a heater device, a steam generator, a steam turbine, a heat transporting fluid, a storage material, and a piping system, see FIG. 1. In order to achieve high efficiency of the heat-to-power cycle, the steam generator should be operated with temperatures of at least 600° C. As a result, the heat storage has to be charged with temperatures higher than 600° C. because of heat losses during operation and, in case of horizontal heat storage, the mixing of the temperature profile inside the heat storage due to natural convection.
Due to thermal losses and the influence of natural convection the temperature of the heat transporting fluid is lower during discharging than during the charging process. This effect is especially relevant for long downtimes of the system. To compensate for this temperature loss, the charge temperature is elevated. The ducting between the thermal storage and the steam generator is stressed by the high temperature of the hot fluid. If it needs to be designed for temperatures higher than 600° C. the costs rise strongly, because high thermal resistant material needs to be used. Hence the possible temperature increase needs to be as low as possible to limit the additional expenditure.
Because the installation cost for thermal energy storage plants needs to be as low as possible in order to be able to make profit, the use of custom products should be kept at a minimum level.
A second effect is the drop of the discharge temperature during the discharge cycle. The discharge temperature drops down after approximately 40% of the discharge time, see FIG. 2. The steam cycle is designed for a certain feed in temperature and if this temperature reduces the gas and water flow need to be controlled to either keep a certain enthalpy flow or to reduce the discharge power. This is possible up to a level where the steam quality is too low for the designed steam cycle. Hence the discharge cycle needs to be stopped.